The present invention relates to a self-cleaning method for filtering dust. In another aspect, the invention relates to an improved method for cleaning tubular fabric filters.
Finely divided particulate is frequently filtered from a dustladen gas stream with a tubular filter constructed of fabric material. The fabric material is usually porous with the average diameter of the pores generally being larger than the diameters of the smallest particles to be filtered. When very clean filters, for example, new filters, are utilized, a portion of the particles pass through the filter and are not recovered. As filtering operations continue, particles accumulate on the filter partially clogging the over-size pores and thereby increasing the effectiveness of the filter. The portion of particles collected by the filter from the gas stream increases with increasing amounts of collected particulate. Eventually, the accumulation of particles on the filter is so heavy that the gases in the gas stream can no longer satisfactorily pass through the filter. At this point it is highly desirable to clean the particles from the filter.
Cleaning of the tubular filter has been usually carried out by reverse flow of gases through the filter wall. This method of cleaning the filter generally either results in inadequate cleaning due to the total collapse of the tubular filter or in uneven cleaning with portions of the filter being made too clean for efficient filtering. The cause of uneven cleaning is that the tubular fabric filter partially collapses in areas between its annular supportive structures thereby forming a venturi-shaped passage or series of venturi-shaped passages along its length. The pressure at the throats of these venturi-shaped passages is lower than the pressure at other points within the tubular fabric filter resulting in a higher pressure differential between the exterior of the tubular fabric filter and the interior of the tubular fabric filter at the area of the venturi than the pressure differential between the outside of the tubular fabric filter and other points within the tubular fabric filter. The higher pressure differential causes a disproportionately large amount of gas to reverse flow through the wall of the tubular fabric filter adjacent to the venturi-shaped passage causing a disproportionate amount of cleaning. Utilization of low back flow pressure to avoid formation of venturi-shaped passages generally results in inadequate cleaning of the filter wall.
Another method by which the prior art teaches that tubular filters can be cleaned is by injecting a pulse of gas along the longitudinal axis of the filter. Utilization of this method tends to shear accumulated particulate cake from adjacent the interior surface of the filter wall. A disadvantage of this method is that particulate accumulations in the interstices of the tubular filter wall are not removed unless, as in U.S. Pat. No. 3,178,868, there is enough slack in the wall of the filter so that the wall of the filter will snap inwardly to dislodge the interstitial accumulations upon passage of the axial pulse. Reverse flow cleaning is not particularly efficient with a slack-walled tubular filter because it causes collapse of the filter.
Because reverse flow gases are best suited for dislodging interstitial accumulations of particulate, and axial pulsed gases best suited for removing caked accumulations of particulate, it is extremely desirable to provide a method by which reverse flow and pulsed cleanings of tubular fabric filters can be most efficiently combined.